1. Field of the Invention
This invention relates to a screw, more particularly to a screw for fastening wooden materials.
2. Description of the Related Art
Referring to FIG. 1, a conventional screw 13 used for coupling overlapping first and second wooden materials 11, 12 includes a head 131, a shank portion 132 extending from the head 131 and having a tip opposite to the head 131, and a helical thread 133 extending helically around the shank portion 132 between the head 131 and the tip. To use the screw 13, a rotating torque is applied to the head 131 by a tool (not shown) so as to drive the shank 132 through the first wooden material 11 and into the second wooden material 12, thereby coupling tightly the first and second wooden materials 11, 12.
Since the first and second wooden materials 11, 12 have strong fiber textures, when the screw 13 is rotated, the helical thread 133 rotates destructively into the first and second wooden materials 11, 12, and cuts through the fiber textures so as to facilitate extension of the shank portion 132 therethrough. However, since the single-direction rotation of the helical thread 133 cannot completely cut the fiber textures of the first and second wooden materials 11, 12, the shavings 14 produced thereof are rough, and are easily tangled around the screw 13, so that the shavings 14 are stuck either inside or at the surface of the first wooden material 11. Thus, not only is a tightening resistance of the screw 13 large and a penetrating force thereof low, but also, the screw 13 cannot completely and tightly interconnect the first and second wooden materials 11, 12. Moreover, when the first and second wooden materials 11, 12 are subjected to an external force, the conventional screw 13 tends to rotate in a reverse direction of the helical thread 133 so as to be loosened.